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Remove:
	HELPER_CACHE_DIR
	HELPER_IMG
	HELPER_KERNEL
	HELPER_INITRD
 	TIMELIMIT
Add:
	TIMEOUT

In configure, remove:
	--enable-network-backend
	--with-helper-img
	--with-helper-kernel
	--with-helper-initrd

This can be used to enable or disable the version check between
snf-image-host and snf-image-helper at the end of the update-helper
script.

Using package caches to create the helper image is dangerous now that
snf-image-helper is automatically retrieved.

The files hosted in this directory will be added in the
/etc/apt/preferences.d directory of the helper image before multistrap
runs.

By default this directory is $(LOCALSTATEDIR)/cache/snf-image/helper

multistrap is more flexible allows as to define more that one
repository to retrieve packages from.

The curl dependency is not optional any more.

The latter is part of gnu coreutils.

snf-image will detect at runtime if pithos backend is installed

snf-image-helper by default uses it's own version of unattend.xml file,
but if UNATTEND variable is specified then it will use the one provided
by snf-image-host.

snf-image-host will make use the external progress monitor program
if the PROGRESS_MONITOR variable is not empty. This way progress
monitor can be enable or disabled by just overwriting the aforementioned
variable in the defaults file.

 * Drop "custom" image format type. The same functionality is provided
   by network-hosted images.
 * Images with id that starts with "http:", "https:", "ftp:", "ftps:"
   are considered to be network hosted and are downloaded via cURL.
 * Images with id that starts with "pithos:" are considered to be
   pithos-hosted.
 * Support for pithos and network hosted images is disabled by default.
   You may enable it when configuring the project  using
   --enable-pithos-backend and --enable-network-backend respectively.

Since curl is needed to find remote file sizes, lets do the whole
downloading with curl.

* Add code to compute the remote filesize
* Check for wget and curl in the configuration phase